With the ongoing development of opto-electronic elements such as active opto-electronic devices (lasers, photodiodes), passive waveguide devices (beam splitters, lenses), and functional optical waveguide devices (optical switches, wavelength filters) there is an increasing demand for links between these optical devices. Single-mode as well as multi-mode fibers linking optical devices or systems serve as perfect transmission media for optical signals. Coupling opto-electronic circuits and fibers is a challenging area to which more and more attention is paid.
Typical approaches provide for optical links between opto-electronic devices, such as edge-emitting laser diodes, and fibers. These approaches are based on the idea to align the optical fibers in a groove etched into a substrate, preferably silicon (Si). By coupling this substrate to an edge-emitting device, or fixing an edge-emitting device on top of this substrate, the emitted light is coupled into the fiber. An overview, of different approaches known, is given by the following articles:
"Permanent Attachment of Single-Mode Fiber Arrays to Waveguides", E. J. Murphy et al., Journal of Lightwave Technology, Vol. Lt-3, No. 4, August 1985, pp. 795-798; PA1 "Self-Adjusted Permanent Attachment of Fibers to GaAs Waveguide Components", H. Kaufmann et al., Electronics Letters, Vol. 22, No. 12, June 1986, pp. 642-644; PA1 "Passive Coupling of InGaAsP/InP Laser Array and Singlemode Fibers Using Silicon Waferboard", C. A. Armiento et al., Electronics Letters, Vol. 27, No. 12, June 1991, pp. 1109-1111. PA1 1. a surface receiving GaAs detector 12, and PA1 2. a silicon substrate 10 with perpendicular groove 14, in particular pyramidal.
These approaches are not suited for the alignment of fibers to surface emitting or surface receiving opto-electronic devices. Specially in the field of high integrated opto-electronic circuits it becomes more and more important to provide for an optical link perpendicular to the substrates surface, which allows dense packaging of multiple devices with separate fiber links. Some approaches, described in the below cited references, are known in the art which provide for a perpendicular link of a fiber to an opto-electronic device.
The article "Silicon Photodetector Structure for Direct Coupling of Optical Fibers to Integrated Circuits" R. W. Ade et al., IEEE Transactions on Electron Devices, Vol. ED-34, No. 6, June 1987, pp. 1283-1288, concerns a silicon photodetector having an integrated fiber-optic coupler (IFOC). This fiber-optic coupler mainly consists of a hole being etched into the top layer of the photodetector, such that an optical single-mode fiber with tapered end can be inserted into this hole. The position of the end facet of the fiber is defined by the diameter of the tapered end and the diameter and depth of the hole. Light, being fed through the fiber, is coupled into the detector via a small air gap.
Another coupling scheme, providing for a link between a single-mode fiber with tapered end and a surface receiving p-n junction diode, is described in the article "Efficient Coupling of Optical Fiber to Silicon Photodiode", O. Baltuch et al., IEEE Electron Device Letters, Vol. 10, No. 6, June 1989, pp. 255-256. The diode is monolithically integrated on a silicon substrate, its top being covered with a contact metallization and a thick layer of spin-on glass (SOG). A hole is etched through the SOG layer and the metallization providing for a window to the diode. A single-mode fiber is aligned to the diode by inserting its tapered end into the hole.
A quasi-monolithic opto-electronic circuit is disclosed in U.S. Pat. No. 4,890,895 comprising an opto-electronic GaAs device 12 being formed on a silicon substrate 10, as illustrated in FIG. 1. This substrate 10 has a groove 14, perpendicular to its main surface 16, such that an optical fiber 15, which is inserted into the groove 14, is aligned perpendicular to the GaAs device 12. A ball lens 13 is situated between fiber end and the GaAs device 12 to improve the coupling efficiency. In addition, a silicon device 11 can be monolithically integrated on the silicon substrate 10. This U.S. patent forms the nearest prior art with respect to the present invention since it shows:
The U.S. patent explicitly relates to systems for opto-electronically interconnecting III-V or II-VI devices grown on silicon substrates. The fiber 15 is aligned to the respective device 12, integrated on the substrate 10, by etching a groove 14 in its back side 16 which extends up to the device 12 to be opto-electronically interconnected. This etch-step is in the above patent specification and hereinafter referred to as back-etching. Before back-etching the silicon substrate 10, a resist mask has to be formed, requiring photolithographic steps, including very precise alignment of the respective photolithographic masks, in addition to the photolithographic steps for the definition of the devices on top of the substrate. By etching the groove to expose the bottom of a device to be interconnected, typical etch damages occur, reducing the reliability and quality of the whole opto-electronic circuit.